1. Field of the Invention
The present invention relates to computer memory devices, and more particularly to a memory module adaptor configured for adapting single memory module sockets to receive a plurality of memory modules of varying memory capacity and speed.
2. Description of the Prior Art
Computers utilize various integrated circuit devices for the storage and retrieval of digital data. These storage devices are generically referred to as memory devices. In the past, a computer's random access memory (RAM) consisted primarily of a plurality of single-bit integrated memory chips connected directly to the computer central processing unit (CPU) motherboard. In order to expand a computer's memory, additional memory chips had to be added directly to the motherboard. Because each memory chip required a certain amount of physical space on the motherboard, the memory capacity of a computer was limited as to the number of memory chips which could be physically added to the motherboard.
As computer speeds and software memory requirements increased dramatically in the 1980's, the need for greater computer memory became evident. Thus, in the mid-1980's, computer manufacturers began providing computers with memory sockets designed to receive memory modules on which memory chips were mounted, and these sockets remain in use today. A typical memory module contains 8 memory chips and a parity chip and is plugged directly into the motherboard memory socket. These memory modules are commonly referred to as single in-line memory modules (SIMMs) or single in-line parallel pins (SIPPs), and the memory sockets as SIMM or SIPP sockets. By plugging in SIMMS to the SIMM sockets, computer users could easily expand the memory capacity of their computers without taking up additional space on the motherboard.
SIMMS and SIMM sockets were originally designed as 30-pin, 8-bit wide memory devices. Computers were manufactured with at least one 30-pin SIMM socket, allowing the user to expand the computer memory by plugging in a 30-pin SIMM to this socket. Often computers were manufactured with several SIMM sockets so that users could periodically add additional memory capacity. When these 30-pin SIMMs and SIMM sockets were first introduced, most computers utilized 8-bit microprocessors. One or a few SIMM sockets were adequate to provide the needed memory for these computers. However, computer manufacturers recently introduced computers with 32-bit microprocessors which enable use of software requiring more memory capacity. In order to provide for the memory requirements of these computers, a greater number of SIMM sockets were required. The increased number of SIMM sockets caused the same space constraints as the previous directly applied memory chips. Thus, although 30-pin SIMMS and SIMM sockets provided a significant improvement over the old directly connected memory chips, increasing computer memory requirements outpaced their capability.
To meet increased memory and speed requirements, computer manufacturers have recently begun to manufacture computers with 72-pin SIMM sockets and higher parallel bit order SIMMS. However, until now, the new 72-pin sockets could not receive 30-pin SIMMs. In order to expand the memory of these new generation computers, users must buy expensive 72-pin memory modules even though they may already own 30-pin SIMMS purchased for other computers. Moreover, newer SIMM sockets require SIMMS with a parallel bit capacity of 32-bits in order to allow communication with the computer CPU. Thus, expensive 8-bit wide 30-pin SIMMS find no utility in a computer designed with a 72-pin SIMM socket and a 32-bit CPU.
Another problem with the prior art is the lack of an adaptor having means to selectively vary the presence detect code. The CPUs of computers with memory module sockets are designed to detect the cumulative memory capacity and speed of the memory modules connected to the computer. In older 30-pin memory modules, the computer CPU scanned the module itself to determine the module's memory capacity and speed. In newer 72-pin memory modules, a digital representation of the value of the memory speed and capacity is encoded in a presence detect code. The computer CPU merely reads this code to determine the memory capacity and speed of the memory modules attached to it.
72-pin memory modules dedicate four of their pins for providing the presence detect code, and the 72-pin memory module receiving sockets dedicate four pin connectors for communicating this presence detect code to the computer CPU. The four pins on the 72-pin memory modules are either in an open state or closed state, representative of a 1 or 0, respectively. The computer CPU reads all four presence detect pins in parallel to determine a 4-bit binary code representative of the amount and speed of the memory connected to the computer. Thus, the presence detect code could have 2.sup.4 or sixteen different values.
In the prior art, the presence detect codes of 72-pin memory modules are fixed by hard wiring the presence detect pins of the modules to either the open or closed states. Since the presence detect pins are hard-wired, the presence detect codes cannot be changed.
Memory module extenders which permit single 30-pin SIMM sockets to receive multiple 30-pin SIMMS are known in the art. However, these prior art extenders cannot adapt a 30-pin SIMM to a 72-pin SIMM socket, nor can they adapt a plurality of 8-bit SIMMs to function as a single SIMM of a higher order bit capacity. Conventional extenders also lack means for providing a variable presence detect code to be read by the computer CPU.
Thus, there exists a need for a memory module adaptor which is configured to adapt a plurality of 30-pin, 8-bit SIMMs to function as a single, higher-order 72-pin SIMM. A need also exists for a memory module adaptor which can receive memory modules of varying memory capacity and speed and communicate to the CPU a variable presence detect code indicative of the cumulative capacity and speed of the SIMMS connected to the adaptor.